RUST IN WHEAT.

Otago Witness, Issue 1994, 12 May 1892, Page 11

RUST IN WHEAT.

In the March number of the Agricultural Gazette of New South Wales Dr Cobb continues his papers on " What' Has Been Found Out in This and Other Countries Concerning Wheat Rust." The previous papers were principally devoted to inquiries into the nature of the rust fungus as made known to us by men of science. The most important and practical portion of the paper under notice is devoted to an examination conducted by Dr Cobb into the physical properties of rust resistant and nonrust resistant wheats. This examination was carried out in conjunction with Mr William Farrar, Lambrigg, Queanbeyan, New South Wales. Mr Farrar has enthusiastically taken up the rust in wheat question, and has devoted a great deal of time to hybiidising wheats with a view of developing rust resistant qualities, and his successes in this direction promise to have an important bearing upon the rust problem in the near future. His experimental plots contained 130 varieties of wheat, and these wero placed at the disposal of Dr Cobb. It is known to all farmers that some varieties of wheat resist rust much better than others, but the reason of their ability to do so has not been clear, and Dr Cobb's researches have thrown a strong light upon this phase of the rust question. The investigations were undertaken because of the almost entire lack of accurate information concerning the coarser and finer differences in structure that undoubtedly exist among wheats. The main objects of the examination were to discover if there were any differences microscopic or otherwise, between the wheats that resist rust and those that do not, and if so, were those differences constant and definable. The value of such information would undoubtedly, be great to the breeders of wheats, and it is to these that we must look for new wheats that possess in a high degree the power of resisting rust, and at the same time are adapted to the conditions which govern wheat growing in Australia. A breeder of wheat can make but slow progress and arrive at very indefinite conclusions if he has to wait for a specially rusty season to test the rust-resisting powers of the varieties which he has bred ; but if he knew beforehand the properties of a rust-resistant wheat he would be able to examine each new variety and predict with some degree of accuracy its resisting power. The first step taken was to measure the thickness of the leaf in a number of varieties with a micrometer caliper. No sooner were the sections prepared than it became evident that the exterior walls of the cuticular cells, that is to say, the walls that form the surface of the leaf were more than twice as thick in some varieties of wheat as in others ; and furthermore, that the rust-resistant varieties were those whoso cuticular cells had the thicker walls. It was also found that the cuticle of the under surface of the flag was invariably thicker than the upper surface, and this subsequently led to a determination to note in the wheat field the relal ive abundance of the rust on the two surfaces of the leaf. Dt Cobb says, "The result was a surprise to us, and at the same time a mortification, for we found that we had fir years been observing wheat rust without seeing one of the most obvious things about it — namely, that there is in most cases little or no rust on the under surface of the flag. After we had satisfied outselves on this point, it still occurred to us , that this absence of rust on the under surface i might be due to position. The lower surface of the leaf faces outward and downward, and is therefore for the most part in shadow and not j exposed to falling spores, while the reverse iB true of the upper surface. Would this account i for the absence of rust on the under surface ? We found it would not. There were wheat plants with upright leaves which showed the same difference between the two surfaces. Again there were many leaves of other wheats so twisted as to have presented for a long time < a reversal of the two surfaces ; the surface usually downward was here upward, yet it was free from rust. Finally we marked an immature leaf, tied it so as to expose the under surface to \ the sun and weather, but the result was the ' same, only the upper surface (although here turned downward) became rusted. The only conclusion to be arrived at was that the lower surface of the flag in rust liable wheats is comparatively free from rust on . account of the different structure of the cuticle on that side of the flag. This was one conclusion, which,' coupled with the... thickness of cuticle observed in rust resistant varieties • led us to believe that the structure of the cuticle in a variety of wheat has'a most important influence in determining iW liability /to rust." The inquiry which naturally suggests itself ia

— How can a thick cuticle cause a variety of wheat possessing it to be rust resistant ? Dr Cobb answers the question by saying that : — 44 First, the cuticle may be so constructed as to prevent the entrance of rast ; second, it might be so constructed that although it permitted rust to enter it would not let it out again — in other words, to fructify so as to disseminate spores. A thick, tough cuticle would doubtless enable a wheat to act in this second way, and we have no longer any doubt that it is a fact that many rust resistant wheats are such because of a thick cuticle acting in this manner. The cuticle of these wheats is studded with breathing pores, differing in no very marked way from those found on the leaves of non-resistant wheats, or at least not differing in any way that would prevent the entrance of rust. The rust accordingly enters and grows inside ; evidence of this can be easily had by examining such a wheat after it has been subject to the influence of rusted wheat growing near by. Thus Ward's Prolific and Cretan, both notably resistant sorts, show under such conditions numerous round discolourations when the flag is held between the eye and a strong light. These whitish or yellowish discolourations mark the places where rust has entered and grown inside the leaf. Where, however, the rust is ready to produce spores and push them through the cuticle, it meets, in these wheats, with a resistance which it is unable to overcome. The rust must, by its growing force, rupture the cuticle before it can ripen and disseminate its spores, but it finds the cuticle of these resistant sorts too tough for it."

With a view of getting further information concerning the structure of the cuticle it was determined to make a scrips of observations on the size of the stromata or breathing pores. It has occurred to more than one observer that the structure of the stromata must have an imp.rtant bearing on the entrance of the promycelium of rust. Dr Cobb set himself to discover first whether a wheat possessed stromata small enough to prevent the entrance of rust, and next could the stromata, even though large enough to admit the promycelial thread, be so situated or so formed as to refuse it admittance. In a general way it was found that on resistant wheats the stromata were smaller and more numerous than on those that did not posses that quality. Nevertheless the smallest stromata observed were large enough to admit' the rust thread. In carrying out these observations, however, a very interesting discovery was made of a hitherto unknown function of the waxy covering so characteristic of certain wheats, especially when they are young. This wax or bloom gives to the plants on which it occurs a glaucous or whitish appearance according to its abundance. It serves the same purpose as the bloom on grapes and many varieties of foliage — namely, to protect the plant from the injurious action of water. Dr Cohb and Mr Farrar, however, discovered that on the wheat plant it had no small influence in keeping out the promycelium of rust. The waxy bloom, when it occurs in abundance on the sheaths of wheat, almost completely covers the surface of the cuticle, being interrupted only at the stromata. Referring to this fact Dr Cobb says: — "When one examities such a sheath under the microscope it is some little time before the stromata are see, po perfectly are they hidden ; their position is indicated only by a very narrow crack in the wax, a crack so narrow that the promycelial threads of rust fail to enter it. We made this observation repeatedly on a wheat known as 816 carre de Sicile rouge, a particularly glaucous wheat, especially when young. We took sheaths of this wheat and germinated on them large numbers of the spores of Puccinia graminis, but we failed, after a long search, to find a single promycelial thread that had gained entrance. The promycelial thread would creep directly across the stromatic oppning, or rather crack in the wax corresponding with the stromatic opening, without the slightest indication of an effort to enter." These observations clearly explain why the sheath and straw of glaucous wheats often remain unaffected by rust, although the flag may be quite rusty. The latter, especially the upper surface, is much less thoroughly protected by the waxy bloom than the sheath. The two most prominent conclusions, indicated by Dr Cobb's paper as beiug arrived at, are that the thickness and the toughness of the cuticle of rust resistant wheats prevent the ripening and dissemination of rust spores, and that the waxy covering of the foliage exercises a most important function in preventing the entrance of the promycelial threads. These are steps in advance, and indioate th:it it may be possible to breed a wheat combining these useful qualities to such a degree that rust would be powerless to injure it: Assuming that the observations are correct, they give a line for breeders to follow, and instead of groping in the dark they may, having a definite object in view, be able to achieve results that are now looked upon as well nigh impossible. — Melbourne Leader.

Miss Jessie Howat, of the Tapanui School, is recommended for the mistress-ship of the public school at Ophir.